[68Ga]Ga-FAPI versus 2-[18F]FDG PET/CT in patients with autoimmune thyroiditis: a case control study

Purpose Radiolabelled fibroblast activation protein inhibitors (FAPIs) are becoming increasingly important for imaging various tumour diseases. However, it is essential to be aware of potential pitfalls. Here, we investigate FAP expression in the thyroid gland in autoimmune thyroiditis (AIT). Methods AIT patients with pathological thyroid uptake on [68Ga]Ga-FAPI PET were compared with glucose metabolism on 2-[18F]FDG PET in terms of SUVmax/SUVpeak/SUVmean/tissue-to-background ratio (TBR), and with a healthy control group. Results Between September 2019 and July 2021, 6 patients presented with a visually increased thyroid uptake and TBR on [68Ga]Ga-FAPI PET. In the retrospective clinical work-up, all patients had known or newly diagnosed AIT. Compared to a matched healthy control group, FAP expression and glucose metabolism were significantly increased ([68Ga]Ga-FAPI (SUVpeak): 7.0 vs. 1.7; p = 0.004/(TBRbloodpool): 6.8 vs. 1.7; p = 0.002; 2-[18F]FDG (SUVpeak): 3.9 vs. 1.4; p = 0.004/(TBRbloodpool): 4.0 vs. 1.2; p = 0.041). However, there was no significant difference in median uptake between [68Ga]Ga-FAPI and 2-[18F]FDG PET (SUVpeak: 7.3 vs. 5.6; p = 0.104). Conclusion Patients with AIT show higher thyroid uptake on [68Ga]Ga-FAPI and 2-[18F]FDG PET. Incidental thyroid uptake is another pitfall in the interpretation of [68Ga]Ga-FAPI PET and should prompt a clinical work-up. Supplementary Information The online version contains supplementary material available at 10.1186/s13550-024-01129-y.


Introduction
Fibroblast activation protein (FAP) is expressed in the stroma of approximately 90% of all epithelial cancers, and is associated with their angiogenesis, migration and proliferation [1].Its expression can be visualised using radiolabelled FAP inhibitors (FAPIs) on positron emission tomography/computed tomography (PET/CT) scans [2].Despite improvements in FAP-directed radioligands [1,2], false-positive uptake may occur in various conditions including acute or chronic inflammation, degenerative lesions and scarring [3].In particular, inflammatory diseases such as pancreatitis and arthritis should be highlighted [4,5], raising the question of whether [ 68 Ga] Ga-FAPI PET/CT may also detect autoimmune thyroiditis (AIT) [6,7], as fibroblast activation plays a role in all three diseases.In clinical practice, AIT is typically diagnosed by clinical symptoms, laboratory parameters and ultrasound [8].
The study aimed to retrospectively evaluate thyroid uptake on [ 68 Ga]Ga-FAPI PET/CT using several semiquantitative parameters in six patients diagnosed with AIT and to compare the results with thyroid uptake on 2-deoxy-2-[ 18 F] fluoro-d-glucose (FDG) PET/CT in order to better evaluate this pitfall in image analysis in the future.

Materials and methods
The patient flow is shown in Fig. 1.This subgroup analysis is part of the ongoing observational study (NCT04571086) at University Hospital Essen.Between September 2019 and July 2021, six patients diagnosed with AIT underwent [ 68 Ga]Ga-FAPI PET/CT for complex oncological diagnoses and provided informed consent.Inclusion criteria were (a) [ 68 Ga]Ga-FAPI PET/CT for tumour staging/restaging, (b) high thyroid uptake on visual assessment of [ 68 Ga]Ga-FAPI PET and (c) age ≥ 18 years.5/6 patients underwent additional 2-[ 18 F] FDG PET/CT.Confirmation of AIT involved laboratory parameters (TSH, fT3, fT4, TPO-Abs), ultrasound findings, and medical history.
A group of six age-, sex-and disease-matched individuals without AIT at the time of [ 68 Ga]Ga-FAPI/2-[ 18 F] FDG PET imaging was assembled for comparison.
Descriptive statistics and individual patient data are reported.Statistical analyses were performed using GraphPad Prism (version 9.1.0;GraphPad Software, San Diego, California, USA) and SPSS (SPSS Statistics version 27.0, IBM, Armonk, New York, USA).SUV max /SUV mean / SUV peak values for [ 68 Ga]Ga-FAPI and 2-[ 18 F]FDG PET were compared using the Wilcoxon test.Mann-Whitney-U test was performed to compare the diseased cohort with the healthy reference group.This retrospective analysis was approved by the local ethics committee (permits no.20-9485-BO/20-9777-BO).

Results
Six female patients with AIT and pathological FAP expression in their thyroid glands on [ 68 Ga]Ga-FAPI PET/CT and six female controls were reviewed.The median age of the diseased population was 56 years (range 33-74 years), and for the control group, it was 57 years (range 39-73 years).
All patients had known (n = 5) or newly diagnosed (n = 1) AIT.The median interval between [ 68 Ga]Ga-FAPI PET/CT and laboratory parameter assessment was 60 days (IQR: 25-68 days).Table 1 provides further details.Two AIT patients underwent ultrasound showing typical signs of chronic thyroiditis (inhomogeneous thyroid parenchyma, normal perfusion).5 patients received thyroid replacement therapy.

Discussion
Our retrospective analysis demonstrated a significant difference in thyroid uptake and TBR on [ 68 Ga]Ga-FAPI PET in patients with AIT, in line could also be identified for 2-[ 18 F]FDG, compared to a healthy control group.AIT is a pitfall for both 2-[ 18 F]FDG [11] and [ 68 Ga]Ga-FAPI PET.
The intense thyroid uptake in both imaging modalities is probably due to different mechanisms: While [ 68 Ga]Ga-FAPI PET primarily represents fibroblasts and thus the fibrotic remodelling processes occurring in AIT [12], comparable to arthritis and pancreatitis [4,5], the increased glucose metabolism on 2-[ 18 F]FDG PET primarily represents inflammation.The superior delineation on [ 68 Ga]Ga-FAPI PET may be attributed to lower background activity.
AIT progresses in several phases, from inflammation to fibrotic processes and scarring.Whether FAP expression differs between these phases and histopathological subtypes of AIT, particularly the fibrous variant, remains an open question.Various patterns of stromal fibrosis have been described, including interfollicular, interlobular, and scar fibrosis [13], which may contribute to the observed variance in FAP expression (SUV max values ranging from 4.0 to 20.0) in our study.It is noteworthy that the sole patient with newly diagnosed AIT showed markedly higher SUV values in comparison to patients with previously known AIT (Additional file 1: Table S1).However, further data is required to prove a potential correlation.
Although our study has limitations, notably the retrospective design and a small patient cohort, it highlights significant differences in FAP expression and glucose metabolism in AIT patients compared to healthy controls.Further research, especially in potential subgroups of AIT, is warranted.

Conclusion
Incidental thyroid uptake is another pitfall in the interpretation of [ 68 Ga]Ga-FAPI PET, and also of 2-[ 18 F]FDG PET.If thyroid uptake is high, additional testing should be performed to avoid misinterpretation.

Fig. 2 1
Fig. 2 Example of thyroid uptake on [ 68 Ga]Ga-FAPI-46 and 2-[ 18 F]FDG PET in patient no. 3. A shows the maximum intensity projection of [ 68 Ga] Ga-FAPI-46 and 2-[ 18 F]FDG PET/CT and axial images of the thyroid.B shows ultrasound findings of the thyroid with inhomogeneous parenchyma and normal perfusion

Fig. 3
Fig. 3 Comparison of thyroid uptake on [ 68 Ga]Ga-FAPI and 2-[ 18 F]FDG PET in patients with AIT and a healthy control group.A presents the comparison of SUV max /SUV peak /SUV mean (median/standard deviation) values of AIT patients and a healthy control group for [ 68 Ga]Ga-FAPI PET.B shows the corresponding 2-[ 18 F]FDG PET results